Since I cannot find the thread dealing with best possible/feasible night fighters for the USAF, and we've had some discussion on it in the other thread, I'd start it here. What kind of a night fighter, using the technology of the day, would've been best for the USA to design/purchase/produce? A better P-70; earlier 'P-38M'; thinner P-61 Black Widow; or maybe to really try produce Mossie under license? A twin fuselage NF? A single-engined, but 2-seat NF? Basically, you have a basic knowledge about what engines, electronics armament were available back in certain years of ww2, and try and have the suitable night fighter in service, that would use those 'ingredients'.
Please, don't try to use too late and best 'ingredients', we need something that would catch kill something else, while there is enough of nocturnal targets.

Think I would have stuck with the Black Widow. But I do like the Mossie. The P-61 was designed from the ground up as a night interceptor and was a pretty good fighter by all standards regardless of the time of day. The upper turret though need not have been a turret though and stayed in a fixed position. The four 20's in the belly were pretty serious fire power by themselves. Having them in the belly should have cut down on the blast blinding that could happen from guns at a more canopy eye level. Just the for what it is worth.

"The saga of the Northrop P-61 Black Widow begins back in August 1940, at the height of the Blitz on London. During this time, the US air officer in London, Lt. Gen. Delos C. Emmons, underwent a briefing in which he was brought up to date on British progress on radar (Radio Detecting and Ranging). Radar had first been developed in Britain in 1936, and British scientists and engineers were at that time working on the early versions of AI (Airborne Interception) radar sets which could be carried aboard airplanes, enabling them to detect and intercept other airplanes in flight without having to rely on ground installations.

At the same time, the British Purchasing Commission that was shopping for aircraft in the USA announced that they urgently required a night fighter that would be capable of stopping the German bombers that were attacking London by night. Such a fighter would have to be able to stay on station above London all night, which meant at least an 8-hour loiter time. In addition, the night fighter needed to have sufficient combat altitude in order to take on the bombers when they showed up.

When General Emmons returned to the USA, he reported that the British had an urgent need for night fighter aircraft, and that American industry might be able to supply that need. A preliminary specification was drawn up by the Emmons Board and was passed on to Air Technical Service Command at Wright Field in late 1940. Because of the heavy weight of the early AI radar and because of the high loiter time required, a twin-engined aircraft was envisaged.

Northrop Chief of Research Vladimir H. Pavlecka happened to be at Wright Field at that time on an unrelated project, and was told of the Army's need for night-fighters. However, he was told nothing about radar, only that there was a way to "see and distinguish other airplanes". He returned to Northrop the next day. On October 22, Jack Northrop met with Pavlecka and was given the USAAC's specification. At this time, no other company was known to be working on night fighters, although at about this time Douglas was starting work on their XA-26A night fighter and the AAC were considering the A-20B as an interim night fighter."

and "The design was revised still further on November 22. The belly turret was deleted, and the crew was changed back to three--pilot, gunner, and radar operator. The pilot sat up front, and the gunner sat immediately behind and above the pilot. The gunner was to operate the turret via remote control, using a special sight attached to a swiveling chair. A "stepped-up" canopy was used to provide a clear field of view for the gunner. The rear fuselage with its clear tail cone provided the radar operator with an excellent rearward view which enabled him to act as a tail gunner if the plane happened to be attacked from astern. Optionally, the dorsal turret guns could be "locked" into the forward-pointing position, so that they could be fired by the pilot. The belly guns were deleted, and four 20-mm cannon were to be fitted in the wings. This design was formalized into Northrop Specification 8A (or NS-8A), dated December 5, 1940."

Now a few mistakes were made but they were basically 'designing in the dark' when it came to the size/volume and weight of the radar equipment, at least for the first few months.
It turns out you could put some of the early radar sets in rather small planes (Defiant) and at least shoot down something.
Coming up with a compromise between the Defiant's endurance and and 8 hour patrol might have allowed for a smaller aircraft also

Then you have the armament question, how much is enough? Four 20mm guns with 6-9 seconds worth of ammo figuring you are lucky to get one intercept and firing opportunity per flight?
Lighter guns and more firing time?
Enough ammo to swan about the sky for 8 hours and performing multiple interceptions while waiting for daylight to land?

Mission requirements are going to drive the design, not designing kool plane and seeing what you can stuff into it or get it to do once it is flying.
Adapting an existing design is another story but has it's limitations as the P-70 shows.

This is going to be a result of personal bias and less knowledge of the other capable aircraft.

But I'd say P-61C, fitted with turbo-superchargers, lose the third crewmember (gunner) and the dorsal turret. Original P-61 entering the war in '43 as I recall.

Allowing the pilot to be the gunner (as a traditional single seater would be) and the radar/radio operator to guide, the extra weight and space of the big gondola along with weight and drag savings the 4 .50's in the dorsal turret can be shaved for significant gain IMO.
In traditional form the P-61 could climb to 30,000 ft in about 13 min IIRC. The addition of turbo superchargers would add about 50mph and 10k additional service ceiling (perhaps 45-46,000 ft ceiling?)

In reality, the P-61 came too little too late... but isn't the "what if?" what this is about?

You have a fast climbing, very fast bird that is a very stable gun platform with 4 20mike cannon. With the most advanced and most capable radar available, large and heavy as it was.

Reading a bit about the P-70, it seems that only 1600 HP versions of the R-2600 were installed - the 1750 HP versions would gave some 100 HP more above 12000 ft. IOW, at ~15500 ft it would be 1300 HP vs. ~1190, as historically. Any better info out there?

The airframe of the A-20 really makes a lot of sense for an earlier NF, but a bit more powerful (at altitude) engines are needed.

I would be fine keeping and refining the P-61, but just to throw out something different, how about the A-26? They did experiment with it as a night fighter(XA-26A), but that was when it had 2,000hp engines. Use the 2,500hp engines from the A-26K (B-26K air force), keep the top turret and add 2x.50's on each side, and you have a heck of a plane. Would it be that much different than the P-61? Maybe not. Just throwing out another option.

Unless we radically change World War 2, the USAAF had no need for nightfighters optimized for nocturnal interception of enemy bombers. Also, since the USAAF did not conduct a significant night bombing campaign itself, USAAF night fighters would probably not be extensively engaged in escort mssions aimed at protecting RAF bombers against German night fighters. The main role of nightfighters for the USAAF would be in long range intruder missions aimed at attacking German night flying aircraft near their bases, hit and run bombing missions. It seems to me that the ideal aircraft for these missions would be a fast, long range fighter-bomber optimized for night operations at low to medium altitudes. Among existing aircraft, perhaps an earlier P-38 night fighting variant, something like the P-82, or license built mosquitos. I would trade heavy cannon armament for bombload and fuel, since these airplanes would not be bomber interceptors. Frankly, it seems to me that the USAAF has far less need to specially designed nigtfighters than either the RAF or Luftwaffe...and I think that nightfighters like the P-61 were probably a waste of money.

Hmm - a P-38 with a 165-300 gal DT plus a 500-1000 lb bomb (or several 250-500 lbs bombs?)? Attack the day fighter bases during the night, so daylight forces have a bit less of LW to confront? Of course, problem would've been how to find the bases deep in Germany?

Unless we radically change World War 2, the USAAF had no need for nightfighters optimized for nocturnal interception of enemy bombers. Also, since the USAAF did not conduct a significant night bombing campaign itself, USAAF night fighters would probably not be extensively engaged in escort mssions aimed at protecting RAF bombers against German night fighters. The main role of nightfighters for the USAAF would be in long range intruder missions aimed at attacking German night flying aircraft near their bases, hit and run bombing missions. It seems to me that the ideal aircraft for these missions would be a fast, long range fighter-bomber optimized for night operations at low to medium altitudes. Among existing aircraft, perhaps an earlier P-38 night fighting variant, something like the P-82, or license built mosquitos. I would trade heavy cannon armament for bombload and fuel, since these airplanes would not be bomber interceptors. Frankly, it seems to me that the USAAF has far less need to specially designed nigtfighters than either the RAF or Luftwaffe...and I think that nightfighters like the P-61 were probably a waste of money.

Website quote;
"To resolve the controversy, Lt. Gen. Carl A. Spaatz, Commander of United States Strategic Air Forces in Europe, ordered a July 5, 1944, flyoff at Hurn, England, pitting the P-61 directly against Vandenberg’s choice, the British Mosquito. Lt. Col. Winston W. Kratz, director of night fighter training in the United States, bet $500 that the Mosquito could outperform the Widow. According to the 422d NFS historian, the competing P-61, “tweaked” to get maximum performance, proved faster at all altitudes, “outturned the Mossie at every altitude and by a big margin and far surpassed the Mossie in rate of climb.” All in all, the historian noted, “a most enjoyable afternoon-Kratz paid off.” The official report concluded that the “P-61 can out-climb the Mosquito due to the ability of the P-61 to operate indefinitely at military power without overheating,” critical to closing on a bogey"

To add a little perspective, Lt.Col.Kratz also had this to say afterward;

"I'm absolutely sure to this day that the British were lying like troopers. I honestly believe the P-61 was not as fast as the Mosquito, which the British needed because by that time it was the one airplane that could get into Berlin and back without getting shot down. I doubt very seriously that the others knew better. But come what may, the '61 was a good night fighter. In the combat game you've got to be pretty realistic about these things. The P-61 was not a superior night fighter. It was not a poor night fighter. It was a good night fighter. It did not have enough speed"

There really isn't much doubt that the Mosquito NF Mk.XIX or NF Mk.30, which were closer contemperaries to the Black Widow, were superior night fighters. At least with respect to performance numbers.

Considering this a bit more the best bet for a quick dirty night fighter is probably trying to stick turbo Allisons onto an A -20 airframe.

The Turbo P-61 has a bit of a problem in that the performance numbers for the P-61C need the R-2800-73 (same engine used in P-47M) engine so as good as that option may be it doesn't do any good until Fall/Winter of 1944.
Using the -21 engine out of the P-47B/C does increase the power at altitude but also picks up the weight of the turbo system (a P-61C picked up about 2000lbs in weight over a P-61B), perhaps an early version with lower powered engines doesn't need the bigger props used on the P-61C? or can save some weight other places?

Simply pulling the turret of the P-61 doesn't change a whole lot, 3-5mph in speed and with about 800lbs for the turret and 500-620lbs for .50 cal ammo (on a 30-31,000lb airplane clean) it does what for climb? Smaller fuselage will help but that takes longer once the drawings are finished for the big fuselage (when do you ditch the the turret in the planning stage?)

P-38 isn't big enough to hold the very early radars without playing with the fuselage although it didn't have to wait as long as it did to get the navy radar pod.

If the Army and Douglas had got off the stump perhaps the A-26 nightfighter could have showed up earlier but you still have the engine problem for high altitude performance and the A-26B empty weighed with a few hundred pounds of a P-61B empty and the P-61 already had two stage engines.

The wright R-2600 seems to have trouble with having a high altitude version although this is inferred by no high altitude version ever making it to production.
Switching to turbo-Allisons on the A-20 gives up take-off power although by not carrying bombs this may help the weight-take off problem. Guns and ammo have to be kept to well under 2000lb though
Since the 1325hp turbo Allison shows up in P-38s in Feb/March of 1942 it seems viable and at any altitude much over 13-15,000ft is going to give more power than the R-2600s. Perhaps the larger engine nacelles of the A-20 (you still need to fit the original wheels/tires) offer space for a better intercooler set up than the wing leading edges of the P-38 so the A-20 night fighter won't have the problem with engine restrictions that some of the mid P-38s had?

late war Mosquito's used the AI Mk X radar ( Mosquito's VII, XIX, and 30) which was identical to (and sometimes built in the US) the SCR-720 used in the P-61. Mossies over the years used a variety of different radars, getting steadily better.

The A-20 nightfighter (P-70) used both the SCR-720 and the SCR-540 (AI Mk. IV - same as used in the Beaufighter), the SCR-540 required less "nose area" because of it's fixed Yagi antenna, so it could theoretically be mounted in a wider variety of airframes. The down-side to this, is the narrower "cone" of detection than the SCR-720.

late war Mosquito's used the AI Mk X radar ( Mosquito's VII, XIX, and 30) which was identical to (and sometimes built in the US) the SCR-720 used in the P-61. Mossies over the years used a variety of different radars, getting steadily better.